Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/tests/test_sequence.py

@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import torch
+
+from vllm.sequence import IntermediateTensors
+
+
+def test_sequence_intermediate_tensors_equal():
+    class AnotherIntermediateTensors(IntermediateTensors):
+        pass
+
+    intermediate_tensors = IntermediateTensors({})
+    another_intermediate_tensors = AnotherIntermediateTensors({})
+    assert intermediate_tensors != another_intermediate_tensors
+
+    empty_intermediate_tensors_1 = IntermediateTensors({})
+    empty_intermediate_tensors_2 = IntermediateTensors({})
+    assert empty_intermediate_tensors_1 == empty_intermediate_tensors_2
+
+    different_key_intermediate_tensors_1 = IntermediateTensors(
+        {"1": torch.zeros([2, 4], dtype=torch.int32)}
+    )
+    difference_key_intermediate_tensors_2 = IntermediateTensors(
+        {"2": torch.zeros([2, 4], dtype=torch.int32)}
+    )
+    assert different_key_intermediate_tensors_1 != difference_key_intermediate_tensors_2
+
+    same_key_different_value_intermediate_tensors_1 = IntermediateTensors(
+        {"1": torch.zeros([2, 4], dtype=torch.int32)}
+    )
+    same_key_different_value_intermediate_tensors_2 = IntermediateTensors(
+        {"1": torch.zeros([2, 5], dtype=torch.int32)}
+    )
+    assert (
+        same_key_different_value_intermediate_tensors_1
+        != same_key_different_value_intermediate_tensors_2
+    )
+
+    same_key_same_value_intermediate_tensors_1 = IntermediateTensors(
+        {"1": torch.zeros([2, 4], dtype=torch.int32)}
+    )
+    same_key_same_value_intermediate_tensors_2 = IntermediateTensors(
+        {"1": torch.zeros([2, 4], dtype=torch.int32)}
+    )
+    assert (
+        same_key_same_value_intermediate_tensors_1
+        == same_key_same_value_intermediate_tensors_2
+    )